Although applicable to arbitrary shell components, being for example in the case of shell components for a spacecraft particularly applicable to components for elongate objects such as rocket stages, space station modules or satellites, the disclosure herein and underlying problem will be described with reference to components of the fuselage shell of an aircraft.
Conventionally, manufacturing of such shell components involves riveting or welding stringers and frame-mounting clips or angles onto a skin panel intended to form a portion of the outer skin of the aircraft shell. In further steps, frames are positioned above the stringers and riveted to the clips. This process permits flexible repositioning of the frame within the plane perpendicular to the aircraft axis, due to the separation of clips and frames at this stage. However, the subsequent application of a large number of rivets in geometrically constrained locations in order to connect the frame in its desired position to the clips renders this process very labour-intensive and difficult to automate. Furthermore, the large number of clips and rivets contributes to the weight of the aircraft due to overlapping of clips and frames, as well as to the cost of the aircraft due to additional assembly required for attaching the clips.
As an alternative, the use of integral frames is known that include an integrated foot section with contact surfaces for contacting the aircraft skin between the stringers, such that separate clips and labour-intensive operations for connecting clips and frames are unnecessary. In integral frames, the contact surfaces of the foot section are separated by so-called mouseholes, which are holes or recesses formed in the foot section at each position where a stringer crosses the frame. During the assembly of an integral frame with a skin-stringer panel, the frame has to be placed on the panel such that the stringers fit into the mouseholes of the frame.
However, because typically the outer skin of an aircraft is curved such that neighboring mouseholes are oriented in different directions, geometrical constraints severely limit or prevent repositioning of the integral frame in the plane perpendicular to the aircraft axis during assembly, to introduce the stringers into its mouseholes, unless the mouseholes are made large and/or the frame is divided into short segments. Larger mouseholes are undesirable because they lead to an increased risk of local bulging in the outer skin. On the other hand, a division of the frame into shorter segments leads to increased weight of the aircraft in addition to complicating manufacturing due to a larger number of and segment connections.